The invention relates to a disc brake and a method for actuating a wet multiple disc brake that is run wet by use of cooling oil, especially for a road vehicle, wherein stationary and rotating radially aligned discs are pressed against one another in the event of a braking operation, closing a release clearance in the process.
Wet multiple disc brakes have advantages in terms of robustness, brake lining service life and environmental pollution over dry brakes, e.g. disc brakes of the type that has hitherto been used predominantly in road vehicles, e.g. in heavy commercial vehicles.
These dry friction brakes are subject to relatively high wear and produce abraded material from the brake, which contributes to not inconsiderable dust pollution, especially fine dust pollution, of the environment.
Moreover, brake lining (pad) wear necessitates corresponding maintenance work, which gives rise to additional operating costs.
In addition, the known disc brakes are prone to faults in the area of construction sites and in off-road use owing to their open construction, and require additional protective measures in terms of enclosures.
In special trucks, and in heavy construction machines, tractors and the like, on the other hand, the primary brake types used are wet multiple disc brakes, in which the heat which arises in association with braking and the abraded brake lining material mentioned is carried away by cooling oil. Such wet multiple disc brakes show little wear and are operated at low temperatures.
Wet multiple disc brakes are usually combined with an axle transmission, and the oil for the axle transmission is therefore also used as a cooling oil for the operation of the multiple disc brake. Although this simplifies the installation of the brake, it also results in considerable disadvantages. For example, the service life of the axle transmission is reduced by the abraded brake lining material in the cooling oil and by the increase in the oil temperature. Since forced circulation through the discs is not provided and heat dissipation from the axle is furthermore limited, the brake exhibits phenomena associated with overheating in the event of very sharp and frequent braking, causing damage to the oil and a consequent risk of severe mechanical damage to the brake and the axle transmission.
Moreover, the high viscosity of the oil between the friction surfaces of the discs, i.e. the oil in the release clearance, gives rise to frictional losses, even when the brake is released, owing to the internal friction of the oil, and these losses lead to permanently increased braking resistance and correspondingly increased energy consumption during the operation of the vehicle.
These frictional losses form a permanently present residual torque that has to be overcome and, in particular, is dependent on the size of the release clearance, that is to say the spacing between the mutually facing friction surfaces of the discs.
Although increasing this release clearance has already been tried, in order by this means to permit the oil to flow out of the release clearance, the release clearance dimensions proposed are not acceptable for relatively fast-moving road vehicles. Thus, for example, one recommendation was to make the release clearance at least 0.2 mm per friction contact, although this was admittedly for multiple disc brakes which are used in heavy construction machines, tractors, tracked vehicles or the like, i.e. in correspondingly slow-moving vehicles.
For a multiple disc brake with, for example, 10 rotating discs, of the type used on road vehicles, the resulting total release clearance is 20 times the individual release clearance, which means a total release clearance of 4 mm. The resulting increase in the length of the response time of the brake is not acceptable, particularly in light of safety considerations, and therefore the increase in the release clearance has not been implemented to the extent desired for operational reasons.
It is the underlying object of the invention to develop a method and a wet multiple disc brake of the type in question in such a way that its operating capability is improved and its operating behavior is optimized.
This object is achieved by a method for actuating a multiple disc brake that is run wet by use of cooling oil, especially for a road vehicle, wherein stationary and rotating radially aligned discs are pressed against one another in the event of a braking operation, closing a release clearance in the process. After the brake is released, the mutually associated discs are separated from one another by an amount such that an oil-free or largely oil-free release clearance is formed. The release clearance is minimized by advancing the discs before a braking operation. A multiple disc brake according to the invention includes a control device provided by which the discs can be adjusted toward a minimized release clearance before a braking operation.
By way of the invention, a maximum release clearance is obtained during free running, with the result that no oil film with the disadvantages described in relation to the prior art is formed between the mutually facing friction surfaces of the discs.
When a braking operation is imminent, readiness for braking is established by reducing the release clearance until the discs are in slight contact.
To reduce the response time that results from the maximum release clearance, processes that precede the actual brake actuation and that can be detected by the brake system and/or signals are detected and evaluated, ensuring that readiness for braking has already been established before the brake pedal is actuated.
For example, the driver's desire for a reduction in the speed of travel can be detected each time the gas pedal is released by means of a pedal value transmitter arranged on the pedal, and the release clearance can be reduced even at this stage in expectation of a subsequent brake actuation.
In this way, the response behavior of the multiple disc brake is actually improved over conventionally operated brakes, despite the large release clearance. Thus, for example, times for transferring the foot from the gas pedal to the brake pedal in the range of from 0.15 to 0.25 seconds are known in passenger vehicles. In the case of heavy commercial vehicles, this time tends to be somewhat longer owing to the longer lever travels.
Using this lead time, it is possible in the case of an electromechanically actuated brake, for example, which requires 50 ms to close a release clearance of about 1 mm, to implement a release clearance of more than 5 mm without increasing the response delay of the brake.
In evaluating the pedal value signal of the gas pedal, it is also possible to analyze the degree of probability of the occurrence of a brake actuation and to derive therefrom different settings of the release clearance as appropriate to the situation.
When the gas pedal is released relatively slowly, which indicates that there is no immediate dangerous situation, it is appropriate to reduce the release clearance merely to a normal size, from 5 mm to 1 mm, for example.
When the speed with which the gas pedal is released exceeds a predetermined value and a subsequent emergency braking operation is thus probable, the release clearance can be completely eliminated, slight contact between the discs being accepted, if appropriate.
As an alternative or complementary measure to the evaluation of the pedal value signal from the gas pedal, the information from an existing distance warning system can be used in accordance with the invention to prepare for a braking operation. If a rapid or impermissible reduction in the distance with respect to a preceding vehicle is detected or an obstacle is suddenly encountered, the available information from the distance warning system can be used to reduce or completely eliminate the release clearance as appropriate to the situation. In the same way, it is also possible to take into account signals from vehicle stability control systems (e.g. ESP).
Given a combination of several signals which make it possible to infer the imminent necessity of a braking operation, e.g. signals from the pedal value transmitter mentioned plus a distance warning signal, it is possible to predict the probability of a braking operation being required with particularly high accuracy and thus to implement the adjustment of the release clearance as appropriate to the situation.
The reduction in the release clearance toward its minimization before the actual braking operation is performed in any situation which suggests a braking operation, and this is then preferably maintained for a minimum period, e.g. 10 seconds. After this predetermined maintenance time or in the event of a renewed actuation of the gas pedal, the release clearance is reset to the original larger size corresponding to the unbraked driving state.
According to an advantageous development of the invention, the oil supply to the discs is interrupted in non-braking phases, and the cooling oil is thus thrown out of the widened release clearances, thereby almost completely eliminating the danger of the fluid friction described.
The maximum possible braking energy during a braking operation is decisive for the dimensioning of the discs. Thus, for example, what is referred to as “heat crack braking” represents a maximum demand for a heavy truck (permitted total weight of 18 tons, two axles). This braking operation is used on a friction dynamometer to simulate the fully laden vehicle being driven downhill at top speed. At a braking torque of 2800 Nm and a speed of travel of 85 km/h maintained for 40 seconds, the braking energy at each wheel is 5.0 MJ. This energy has to be temporarily stored in the rotating discs directly absorbing the frictional heat since heat dissipation via the coolant takes a considerably longer time.
In these circumstances, the thermally active mass of the rotor system must be large enough to ensure that the temperature of the components does not exceed 250° C. in order to avoid damage to the oil.
In the example mentioned, the required mass of the rotor discs is 40 kg when the stationary discs, i.e. the stator discs, are fitted with organic linings since the insulating effect of the latter hinders the absorption of heat into the disc. Between brake actuations, heat is dissipated principally by the cooling oil.
The design of the cooling system is determined by the average braking heat that can arise in the event of successive braking operations. For design, use is made of test programs which have proven their worth in previous brake development.
In the case of the two-axle trucks mentioned with a permitted total weight of 18 tonnes, these are, for example, as follows:
Energy per brakeEnergy per hr. drivingoperationtimeHeat crack test (2.85.0 = MJ25 MJ/hkNm; 85 km/h; 40 s,cycle time 12 min)Drastic braking test1.55 MJ18.6 MJ/h(stops from 90 km/h,cycle time 5 min)Hot series (600.67 MJ20.1 MJ/hsuccessive brakingoperations at 2 minintervals)Rossfeld test0.63 MJ69.2 MJ/h(mountain descent asmisuse test; 1030 maltitude difference; 30km/h; 22 min drivingtime; about 110 brakeoperations)
The conditions of the Rossfeld test are decisive for the dimensioning of the cooling system. This gives a cooling capacity requirement of about 20 kW per brake. In this extreme test, the oil supply is active for the entire duration of driving.
For the necessary cooling down time, the heat crack test in turn forms the maximum demand. The large amount of heat absorbed necessitates a cooling down time of about 6 min to allow cooling down to 50° C.
In the case of slight adaptive braking, which accounts for more than 90% of all braking activity, only a short oil supply run-on time is required, thus making it possible in normal operation to avoid the frictional losses caused by the cooling oil.
In the case of an oil pump driven by an electric motor, the oil supply can be interrupted by switching off said pump. On the other hand, valve control is necessary if, even after the oil has run out of the brake housing, the cooling oil that is then in the reservoir is to be cooled down still further. In this case, a multi-way valve directs the oil flow into a cooling circuit from the reservoir to a heat exchanger or from the heat exchanger to the reservoir. In this way, the non-braking phases can be used for improved cooling without causing additional frictional losses in the brake.
The switching off of the oil pump and/or control of the oil supply can be performed as required by the existing brake control system in accordance with the braking activities and the resulting braking heat to be dissipated.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawing.